Semiconductor laser producing method

Details Semiconductor laser producing method Semiconductor laser producing method

2001-06-21

2003-09-30

Chaudhari, Chandra (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Including integrally formed optical element

C438S040000

Reexamination Certificate

active

06627472

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser used in an optical disk device such as a CD-R/RW drive, DVD-RAM drive, MD drive or the like.
2. Description of Related Art
In an optical recording device, it has been intended to improve the recording speed. For example, a CD-R drive having a so-called 12-time recording speed is put into practical use. In such an optical recording device increased in recording speed, it is required to invoke a high-output laser light in an instant. As laser which can satisfy such requirements, there is known a ridge-type semiconductor laser using a compound semiconductor.
FIG. 1A
to
FIG. 1E
are schematic section views illustrating the steps of producing a ridge-type semiconductor laser using a compound semiconductor material.
Successively formed on a substrate
1
are a lower clad layer
2
, an active layer
3
, an upper first clad layer
4
, an etching stop layer
5
, an upper second clad layer
6
, and a contact layer
7
. Further formed on the contact layer
7
is a mask layer
8
, on which a resist pattern
9
is then formed (FIG.
1
A). Each of the semiconductor portions is made of a GaAs-type material, and the mask layer
8
is generally made of SiO
2
or SiN.
Then, the mask layer
8
is etched according to the pattern of the resist
9
(FIG.
1
B). After the resist
9
is removed, the contact layer
7
and the upper second clad layer
6
are etched in the form of a ridge. In the following description, both the upper second clad layer
6
and the contact layer
7
etched in the form of a ridge, are called a ridge section
12
. The etching stop layer
5
is resistant to the etching medium. This prevents the etchings top layer
5
and the layers thereunder from being etched (FIG.
1
C).
Then, light confining layers
10
are grown selectively at both sides of the ridge section
12
(FIG.
1
D). At this time, no light confining layer
10
should be formed on the mask layer
8
. When the light confining layers
10
are made of GaAs, triethylgallium is used as a gallium feeding raw material and a MOCVD (Metal-Organic Chemical Vapor Deposition) method is used, thus causing the light confining layers
10
to be selectively grown.
After the mask layer
8
is removed, a cap layer
11
is grown (FIG.
1
E).
In the semiconductor laser having the arrangement above-mentioned, to confine the laser light in the vicinity of the active layer
3
, each of the refractive index of the lower clad layer
2
and the refractive index of the upper first clad layer
4
must be lower than the refractive index of the active layer
3
.
GaAs absorbs light. Accordingly, when the light confining layers
10
are made of GaAs, it is required to increase the thickness of the upper first clad layer
4
to prevent the GaAs of the light confining layers
10
from absorbing light. This increases the laser operating electric current, resulting in failure in acquirement of a high-output laser light.
SUMMARY OF THE INVENTION
In view of the foregoing, the inventor of this application got the idea that the light confining layers
10
are made of an AlGaAs-type material less in light absorption to enable the upper first clad layer
4
to be thinner to increase the oscillation efficiency, thus obtaining a high-output laser light. Then, he tried to form the light confining layers
10
having a composition of Al
y2
Ga
(1-y2)
As with the use of triethylgallium as the gallium feeding raw material. As a matter of fact, it was found that the component material of the light confining layers
10
deposited also on the mask layer
8
, and that the light confining layers
10
could not selectively be grown in the desired manner. It is noted that such deposits of the component material of the light confining layers
10
will be an obstacle to removal of the mask layer
8
.
This problem may be solved by supplying a corrosive chlorine gas or the like at the time when the light confining layers
10
are selectively grown. However, this causes a variety of another trouble relating to handling of corrosive gas.
Further, to increase the light confining effect to prevent the laser light from spreading transversely in the vicinity of the active layer
3
, the light confining layers
10
are preferably made of Al
y2
Ga
(1-y2)
As of which refractive index is lower than that of the upper second clad layer
6
. In an AlGaAs-type crystal, its refractive index is lower as the Al concentration is higher. Accordingly, the Al concentration of the light confining layers
10
is preferably higher than that of the upper second clad layer
6
. When there is used an AlGaAs-type semiconductor, the upper second clad layer
6
generally has a composition of Al
x3
Ga
(1-x3)
As (0.3<x
3
<0.7). Accordingly, the light confining layers
10
preferably have a composition of Al
y2
Ga
(1-y2)
As(0.4<y
2
<1.0)
It is an object of the present invention to provide a semiconductor laser producing method capable of selectively growing, in the desired manner, light confining layers having an AlGaAs-type composition.
In other words, the specific object of the present invention is to provide a highly efficient semiconductor laser producing method.
A method of the present invention comprises the steps of: successively laminating, on a compound semiconductor substrate, a lower clad layer, an active layer, and an upper first clad layer; forming, on the upper first clad layer, an upper second clad layer in the form of a ridge; and selectively growing, at each side of the ridge-shape upper second clad layer, a light confining layer with the use of a III-group element feeding raw material comprising a III-group element compound having a methyl group (preferably without use of a III-group element compound having an ethyl group).
For example, when there is used a III-group element feeding raw material comprising a III-group element compound having a methyl group for forming the light confining layers according to an MOCVD method or the like, the light confining layers can successfully be grown selectively at the sides of the ridge-shape upper second clad layer. Further, according to the method of the present invention, it is not required to use corrosive chlorine gas when forming the light confining layers.
Preferably, the light confining layers have a composition of Al
y2
Ga
(1-y2)
As (0<y
2
<1.0).
When the light confining layers are composed of Al
y2
Ga
(1-y2)
As (0<y
2
<1.0) less in light absorption, the upper first clad layer can be reduced in thickness, thus enabling the semiconductor laser to supply a high-output laser light. For selectively growing, at the sides of the ridge-shape upper second clad layer, the light confining layers having the composition range above-mentioned, it is effective to use a III-group element feeding raw material comprising a III-group element compound having a methyl group.
More preferably, the light confining layers have a composition of Al
y2
Ga
(1-y2)
As (0.4<y
2
<1.0).
To prevent the laser light in the vicinity of the active layer from spreading transversely such that the laser light is confined in the area under the ridge-shape upper second clad layer, the refractive index of the light confining layers must be lower than that of the upper second clad layer. In an AlGaAs-type crystal, its refractive index is lower as the Al concentration is higher. In an AlGaAs-type semiconductor laser, the upper second clad layer generally has a composition of Al
x3
Ga
(1-x3)
As (0.3<x
3
<0.7). Accordingly, the light confining layers preferably have a composition of Al
y2
Ga
(1-y2)
As (0.4<y
2
<1.0). According to the method of the present invention, the light confining layers having such a composition can be formed.
An embodiment of the method of the present invention comprises the steps of: forming, on a GaAs substrate, a lower clad layer of Al
x1
Ga
(1-x1)
As; forming an active layer on the lower clad layer; forming, on the active layer, an upper first clad layer of Al
x2
Ga
(1-x2)
As; forming, on the upper first clad

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